The present invention relates to a semiconductor laser device having a protective coating with a high-reliability formed on an end surface, and to a method for manufacturing the same.
As shown in FIG. 5, most semiconductor laser devices are composed of, for example, protective coatings 43a and 43b, each having an identical reflectance, formed on light emitting end surfaces 41a and 41b of a GaAs laser chip 4. Reference numeral 42 denotes an active layer of the laser chip 4. In the case where the protective coatings 43a and 43b are composed of Al2O3 in FIG. 5, if a refractive index of the Al2O3 film is set to be 1.60 while a refractive index of the laser chip 4 is set to be 3.50, a reflectance of the protective coatings 43a and 43b corresponding to a coating thickness d varies as shown in FIG. 6 (provided that a laser emission wavelength xcex=7800 xc3x85).
FIG. 6 indicates that regardless of the coating thickness d of the protective coatings 43a and 43b, the reflectance thereof is smaller than that of the case without the protective coatings 43a and 43b (i.e. the reflectance of the light emitting end surfaces 41a and 41b). The reflectance becomes smallest when an optical coating thickness (refractive index nxc3x97coating thickness d) is an odd multiple of xcex/4, while the reflectance becomes approximately equal to that in the case without the protective coatings 43a and 43b when the optical coating thickness is an integral multiple of xcex/2. This is because the refractive index (1.60) of the protective coatings 43a and 43b is smaller than the refractive index (3.50) of the GaAs laser chip 4.
Contrary to this, in the case where the refractive index of the protective coatings 43a and 43b is larger than the refractive index of the GaAs laser chip 4 (for example, if such material as Si is used as the protective coatings 43a and 43b, the reflectance thereof is larger than that in the case without the protective coatings 43a and 43b, regardless of the coating thickness), the reflectance becomes largest when the optical coating thickness is an odd multiple of xcex/4, while the reflectance becomes approximately equal to that in the case without the protective coatings 43a and 43b when the optical coating thickness is an integral multiple of xcex/2.
In the case of high output semiconductor laser devices with optical output as high as 20 mW or more, as shown in FIG. 7, for increasing optical output Pf from the side of a main emitting end surface (front end surface), the reflectance of the protective coating 43a on the side of the main emitting end surface 41a is generally set lower than that in the case without the protective coating 43a, while the reflectance of the protective coating 43b on the side of a rear emitting end surface 41b is set higher than that in the case without the protective coating 43b. 
For example, the reflectance of the protective coating (Al2O3) 43a on the main emitting end surface 41a is set to be approx. 15% or less. This reflectance is obtained with the coating thickness of approx. 700 xc3x85 to 1600 xc3x85.
The protective coating 43b on the rear emitting end surface 41b, if composed with use of a film having a refractive index larger than that of the laser chip 4, is not capable of providing a sufficiently high reflectance as a single layer. Accordingly, an Al2O3 film with a thickness of xcex/4 as a first layer 431 and a third layer 433, and an amorphous Si with a thickness of xcex/4 as a second layer 432 and a fourth layer 434, are laminated. Then finally, an Al2O3 film with a thickness of xcex/2 as a fifth layer 435 is laminated. This makes it possible to form a protective coating 43b having a reflectance as high as approx. 85% or more. It is noted that reference numeral 43 denotes an active layer.
Description will now be given of a method for forming protective coatings 43a and 43b having the above-described reflectance on light emitting end surfaces 41a and 41b of a semiconductor laser chip 4.
First, after one side of an n-GaAs substrate is polished, a p-electrode (comprising an ohmic electrode and a bonding electrode) is formed by evaporation or sputtering. A photomask is made thereon and, then, the p-electrode is patterned by etching.
Subsequently, after the other side of the n-GaAs substrate is polished, an n-electrode (comprising an ohmic electrode and a bonding electrode) is formed by evaporation or sputtering according to the above procedures. After that, the electrodes and the substrate are alloyed at appox. 400 to 500xc2x0 C.
Next, as shown in FIG. 8, a cleavage line 49 is formed by scribing extensively disposed between an electrode 44 of an arbitrary element in a semiconductor laser wafer 5 and an electrode 44xe2x80x2 of an adjoining element in direction orthogonal to an emitting section (channel) 42. Then, as shown in FIG. 9, the semiconductor laser wafer 5 is cleaved and divided into a plurality of laser bars (bar-shaped laser chips) 51.
Next, as shown in FIG. 10, a plurality of the laser bars 51 obtained by dividing are loaded in a laser bar fixing device 6 such that the electrodes 44 face towards the same side. All the laser bars 51 should be loaded so that all the emitting end surfaces 41a are positioned on the same side and, therefore, all the emitting end surfaces 41b are positioned on the same side. Next, on the emitting end surfaces 41a and 41b of laser bars 51 loaded in the laser bar fixing device 6, a protective coating having a specified reflectance is formed generally by using a vacuum evaporator 7 schematically shown in FIG. 11. The vacuum evaporator 7 is provided with a vapor source 72, a holder 73 for holding a plurality of the laser bar fixing devices 6, and a crystal oscillator 74 for monitoring the thickness of evaporated films, all in a chamber 71.
Following description discusses procedures of forming the protective coating. First, in the case for evaporating a protective coating onto the emitting end surface 41a, the holder 73 is disposed such that the emitting end surface 41a of a laser bar 51 faces the vapor source 72 side as shown in FIG. 11. Then, the chamber 71 is evacuated through a duct 75. After a specified degree of vacuum is obtained, an evaporation material 76 put in the vapor source 72 is heated and evaporated by electron beams and the like so that a protective coating is evaporated onto the emitting end surface 41a of the laser. After evaporation is completed, the holder 73 is then rotated 180xc2x0 for evaporating a protective coating onto the emitting end surface 41b according to the above procedures.
Here, a forming speed (evaporation rate) for forming a protective coating on the both light emitting end surfaces 41a and 41b is controlled so as to be approximately constant till completion of evaporation. The evaporation rate is in this case controlled with a heating temperature. In the case of electron beam evaporation, therefore, the evaporation rate may be controlled with the intensity of electron beams. It is well known that in the case of resistance heating, the evaporation rate is controlled with an amount of electric current passed through a resistor. The evaporation rate is generally set in a range between several xc3x85/sec to 30 xc3x85/sec in the case of the evaporation material of Al2O3 Evaporation is conducted while coating thickness is monitored with use of the crystal oscillator 74. Evaporation is terminated when a prescribed coating thickness is obtained.
Although not shown in FIG. 7, in the case of a high output type semiconductor laser device, a low reflecting protective coating 43a (having a reflectance of approx. 15% or less) is formed on the side of the main emitting end surface 41a, and then a multilayered high reflecting protective coating 43b is formed on the side of the rear emitting end surface 41b. The multilayered high reflecting protective coating 43b is composed of a laminated structure made up of: a first layer 431 and a third layer 433, each consisting of an Al2O3 film with a thickness equal to xcex/4; a second layer 432 and a fourth layer 434, each consisting of an Si film with a thickness equal to xcex/4; and a fifth layer 435 consisting of an Al2O3 film with a thickness equal to xcex/2. For evaporation of this film, Al2O3 and Si are mounted on the vapor source 72 as evaporation materials 76. Then the first layer 431, the third layer 433, and the fifth layer 435, each consisting of an Al2O3 film are evaporated through irradiation of the evaporation material Al2O3 with electron beams, and the second layer 432 and the fourth layer 434, each consisting of an Si film, are evaporated through irradiation of the evaporation material Si with electron beams.
However, the above-stated prior semiconductor laser devices have a following problem. In forming protective coatings 43a and 43b of laser chip 4 by evaporation, an oxide (Al2O3), that is a material of the protective coatings 43a and 43b, is decomposed to generate oxygen immediately after start of evaporation process, which increases partial pressure of oxygen molecules. The oxygen, colliding or bonding with end surfaces 41a and 41b of the laser chip 4, is highly likely to cause damage to the end surfaces 41a and 41b. Further, in the case where active layer 42 of the laser chip 4 and vicinity layers thereof contain aluminum, the damage is considered to be larger. If thus-fabricated semiconductor laser device is operated with high output, necessary reliability may not be provided.
For high output type semiconductor laser devices, as shown in FIG. 12, there has been proposed a method for forming a protective coating 43a on the side of a main emitting end surface 41a of a laser chip 4 utilizing high thermal conductivity of Si, in which an Si film 436 having high thermal conductivity is formed first and then a low reflecting protective coating 437 is formed (JP-A 1-289289). In the drawing, reference numeral 43b denotes a multilayered high reflecting protective coating on the side of a rear emitting end surface 41b composed of a first layer 431, a second layer 432, a third layer 433, a fourth layer 434, and a fifth layer 435, and reference numeral 42 denotes an active layer.
In this example, heat generated in the vicinity of the main emitting end surface 41a by light emission of the semiconductor laser device is efficiently discharged by the Si film 436, which controls deterioration of the semiconductor laser device caused by long term supply of current. The Si film 436 has a film thickness of around xcex/4 (approx. 532 xc3x85 in an embodiment).
Further, according to the high output type semiconductor laser device disclosed in the JP-A 1-289289, in forming protective coating 43a on the main emitting end surface 41a, the Si film 436 having high thermal conductivity is formed first for increasing heat dissipation to improve reliability. In this case, the Si film 436 is firstly formed, which is free from generation of oxygen due to decomposition of the material in the process of evaporation, thereby enabling creation of a coating in the vicinity of the emitting end surface 41a of the laser chip 4 immediately after start of evaporation process under conditions of low partial pressure of oxygen. Therefore, an effect of controlling the above-stated damage in the vicinity of the emitting end surface 41a may be achieved.
In this case, however, the Si film 436 has a thickness as high as approx. 532 xc3x85 (almost equal to xcex/4), which may cause leakage current in the Si film 436 (light emitting end surface), and may affect oscillation characteristics of the semiconductor laser device.
JP-A 2000-361037 discloses a semiconductor laser device capable of controlling generation of leakage current in the vicinity of the light emitting end surface by setting the thickness of the Si film to 40 xc3x85 or less.
According to the above constitution, before an oxide is formed as a protective coating, an Si film is formed, which is free from generation of oxygen due to decomposition. Consequently, creation of the coating is conducted immediately after start of Si film forming under conditions of low partial pressure of oxygen, which prevents oxygen with high energy from colliding or boding with the light emitting end surface. Further, if oxygen is decomposed in the process of oxide forming and so the oxygen partial pressure increases, collision or bonding of the oxygen with the light emitting end surface is prevented. Thus, the damages given to the light emitting end surface in the process of protective coating formation are controlled.
Here, if the semiconductor laser chip has an active layer including Al, the damages given to the light emitting end surface is effectively controlled.
In addition, the Si film has a film thickness as small as 40 xc3x85 or less. This reduces generation of leakage current in the Si film or on the light emitting end surface, thereby preventing negative influence on the oscillation characteristics.
Thus, in the case where a protective coating having a specified reflectance is formed on a light emitting end surface of a semiconductor laser chip by evaporation, it becomes possible to reduce damages given to the end surface of a laser chip in creation of a protective coating, and to control generation of leakage current in the vicinity of the end surface of the laser chip. Therefore, it becomes possible to improve reliability of a laser device.
However, in the case where the Si film 436 is formed on the main emitting end surface 41a, as shown in FIG. 13, it is possible that Au in the electrode metal film 45 and Si in the protective coating 437 on the end surface react and Au diffuses into the end surface of the main emitting end surface 41a. 
If a region where Au and Si react (Au/Si reaction region 438) exists over an emission point 42a (an active layer part) as shown in FIG. 13, it may cause leakage current through the diffused Au (in the end surface), and may affect oscillation characteristics of the semiconductor laser device.
According to the present invention, in forming a semiconductor laser device, an electrode comprising Au is patterned so that the electrode does not exist in the vicinity of a light emitting end surface. Thereby, even when an Si film is formed on the light emitting end surface, the Si film is prevented from contacting with the light emitting end surface.
In addition, after patterning the electrode, an insulating film (a silicon nitride film) is formed on the electrode for preventing the Si in the protective coating on the end surface from contacting with Au in the electrode, even when the Si film contacts with a surface of the electrode.
More specifically, the present invention provides a semiconductor laser device, comprising:
an electrode metal film formed on a crystal surface of a semiconductor substrate; and
an Si film formed on a light emitting end surface;
wherein an interval of a specified distance is provided between the electrode metal film and the light emitting end surface.
The semiconductor laser device according to the present invention is useful, in particular, when the electrode metal film comprises Au.
Further, in the semiconductor laser device according to the present invention, an insulating film is further formed on the above electrode metal film. Particularly, in the semiconductor laser device according to the present invention, the insulating film comprises SiNx, Al2O3, SiO2 or TiO2.
In the semiconductor laser device according to the present invention, the interval provided between the electrode metal film and the light emitting end surface is 1 xcexcm or more, preferably 3 xcexcm or more, more preferably 6 xcexcm or more, and most preferably 11 xcexcm or more.
The present invention also provides a method for manufacturing a semiconductor laser device, comprising:
a step of forming a p- (or an n-) electrode by forming an ohmic electrode metal film on one crystal surface of a semiconductor laser wafer, forming a bonding electrode metal film thereon, and patterning the electrode metal films;
a step of forming an n- (or a p-) electrode by forming an ohmic electrode metal film on the other crystal surface of the semiconductor laser wafer, forming a bonding electrode metal film thereon, and patterning the electrode metal films;
a step of dividing the semiconductor laser wafer into individual laser bars, each bar comprising a plurality of semiconductor laser chips by scribing cleavage lines on the semiconductor laser wafer;
a step of forming an Si film on a light emitting end surface which appears in the step of dividing the semiconductor laser wafer; and
a step of forming a protective coating on the Si film.
In particular, the present invention also provides a method for manufacturing a semiconductor laser device, comprising:
a step of forming a first type of electrode by forming an ohmic electrode metal film on one crystal surface of a semiconductor laser wafer, forming a bonding electrode metal film thereon, and patterning the electrode metal films;
a step of forming a second type of electrode by forming an ohmic electrode metal film on the other crystal surface of the semiconductor laser wafer, forming a bonding electrode metal film thereon, and patterning the electrode metal films;
a step of dividing the semiconductor laser wafer into individual laser bars, each bar comprising a plurality of semiconductor laser chips by scribing cleavage lines on the semiconductor laser wafer;
a step of forming an Si film on a light emitting end surface which appears in the step of dividing the semiconductor laser wafer; and
a step of forming a protective coating on the Si film,
wherein the first type of electrode and the second type of electrode are different and are either a p-electrode or an n-electrode.
The method for manufacturing the semiconductor laser device according to the present invention is useful, in particular, when the electrode metal film comprises Au.
Further, in the method for manufacturing the semiconductor laser device of the present invention, the electrode metal films are patterned so that an interval between adjoining two p-electrodes and/or an interval between adjoining two n-electrodes are 2 xcexcm or more, preferably 6 xcexcm or more, more preferably 12 xcexcm or more, and most preferably 22 xcexcm or more.
That is, according to the method for manufacturing the semiconductor laser device of the present invention, since adjoining two electrodes are disposed on a semiconductor laser wafer at an interval of the above described distance, it becomes possible to eliminate generation of leakage current in the vicinity of the laser end surface and, thus, to manufacture a semiconductor laser device having an improved reliability as a laser device, even when errors occur in the step of patterning an electrode on a semiconductor laser wafer, in the step of scribing the semiconductor laser wafer, or in the step of dividing the semiconductor laser wafer into individual laser bars.
Further, in the method for manufacturing the semiconductor laser device according to the present invention, an insulating film is formed on the electrode metal film prior to forming the Si film. In this case, the insulating film comprises SiNx, Al2O3, SiO2 or TiO2, and preferably the insulating film comprises SiNx.
In addition, the method for manufacturing the semiconductor laser device according to the present invention further comprises a step of alloying the semiconductor laser wafer and the ohmic electrode after the step of forming a p- (or an n-) electrode and the step of forming an n- (or a p-) electrode.
For example, after a p- (or an n-) electrode is patterned, a p-ohmic electrode is alloyed with the semiconductor laser wafer by heating the wafer at 400 to 500xc2x0 C. and, after an n- (or a p-) electrode is patterned, an n-ohmic electrode is alloyed with the semiconductor laser wafer by heating the wafer at 400 to 500xc2x0 C.